1. Field of the Invention
The present invention relates to an optical fiber base material ingot having excellent ingot roundness and core portion eccentricity, and a production method which can produce the optical fiber base material ingot in a short period of time.
2. Description of the Related Art
An optical fiber base material ingot is produced through steps of dehydration, sintering and the like, for example, after depositing a clad portion on a surface of starting core member through the OVD (outside chemical vapor deposition). As generally known methods for improving production rate of the step of depositing a soot on a surface of starting core member in the OVD method, there can be mentioned use of a larger diameter of burner for supplying a material gas, which is hydrolyzed in oxyhydrogen flame and deposited as glass microparticles, use of an increased number of such a burner, and the like. When a larger diameter of material gas supplying burner is used for improving the production rate, there arises a problem of extremely bad deposition efficiency of the soot in an early stage of the soot deposition on the surface of starting core member. Furthermore, when a plurality of burners having a larger diameter are used, there arises a problem that intended good deposition efficiency cannot be obtained due to interference of flame. On the other hand, when an increased number of burners is used, irregularities are formed on the surface of the deposited soot because there are a plurality of burners. In particular, when the deposition is performed at a high speed by increasing the amount of material gas, that phenomenon disadvantageously appears notably. Consequently, an optical fiber obtained does not possess good optical characteristics, in particular, desired cutoff wavelength or desired scattering characteristics in a single mode optical fiber. As a method for removing the irregularities on the base material ingot surface, grinding of the base material ingot surface has been known (Japanese Patent Unexamined Publication [KOKAI] No. 2-212328). However, the conventional grinding process has disadvantageously required a long period of time. In addition, though the surface irregularities are removed, the core portion of base material ingot obtained by this method is not necessarily positioned in the center portion of the base material ingot. As a result, the obtained optical fiber causes problems concerning optical characteristics, such as splicing loss in the fiber fusion connection process when laying optical fiber cables.
The present invention has been completed in view of the aforementioned problems, and its major object is to provide an optical fiber base material ingot having excellent ingot roundness and core portion eccentricity, and a method capable of producing such an optical fiber base material ingot in a short period of time. The present invention has been completed in order to achieve the aforementioned object, and provides a method for producing an optical fiber base material ingot comprising depositing a soot on a starting core member, and subjecting the soot to dehydration, vitrification by sintering, and then cylindrical grinding, characterized in that the cylindrical grinding is performed while a core portion central axis is brought into line with a rotation axis of cylindrical grinding machine.
By performing the cylindrical grinding while the core portion central axis is brought into line with the rotation axis of cylindrical grinding machine as in the aforementioned method, the obtained ingot can have excellent roundness without irregularities and, in addition, it becomes possible to exactly position the core portion at the center of the ingot. An optical fiber obtained from the ingot was found to have good optical characteristics, in particular, a desired cutoff wavelength and scattering characteristics and the like in a single mode optical fiber as a result of the above characteristic. In addition, it was also found that the conventional problems concerning optical characteristics, such as splicing loss in the fiber fusion connection process when laying optical fiber cables, can be solved. In the above method, it is desirable that the core portion central axis is brought into line with the rotation axis of cylindrical grinding machine by detecting the position of core portion. By detecting the position of the core portion, the core portion central axis can accurately and easily be brought into line with the rotation axis of cylindrical grinding machine. Further, it is desirable that, in the cylindrical grinding, finishing grinding is performed after at least rough grinding is performed. By dosing so, an extremely smooth ingot surface can be obtained within a short period of time.
Furthermore, by performing the cylindrical grinding through multi-edge grinding, an extremely smooth ingot surface can be obtained within a further shortened period of time. An optical fiber base material ingot of the present invention obtained by the aforementioned method has an extremely smooth ingot surface. As a result, an optical fiber obtained from the optical fiber base material ingot of the present invention has good optical characteristics, in particular, a desired cutoff wavelength, scattering characteristics and the like in a single mode optical fiber. In addition, the optical fiber base material ingot of the present invention has excellent core portion eccentricity. Consequently, an optical fiber obtained from the base material can solve the conventionally observed problems concerning optical characteristics, such as splicing loss in the fiber fusion connection process when laying optical fiber cables. The method for producing an optical fiber base material ingot of the present invention requires extremely shorter production time compared with conventional production methods. Moreover, the optical fiber base material ingot produced by the method for producing an optical fiber base material ingot of the present invention correctly has a desired core/clad diameter ratio, and an extremely smooth ingot surface. Consequently, the obtained optical fiber has good optical characteristics, especially a desired cutoff wavelength and desired scattering characteristics in a single mode optical fiber. Furthermore, the optical fiber base material ingot of the present invention has a core correctly located at the ingot center. As a result, an optical fiber obtained from it is free from the problems concerning optical characteristics, such as splicing loss observed in fusion connection operation when laying the optical fiber cable.